1. Field of the Invention
The invention relates to an implantable, bioresorbable vessel wall support, in particular a coronary stent.
2. Background Art
As for the background of the invention, so-called "vessel wall supports" or "stents"--as they are called in the technical language--are used in the therapy of stenoses, i.e. pathologically constricted passages of a coronary vessel. To this end, such a coronary stent is introduced transvenously into the human body by means of a catheter and delivered through the coronary system to the treatment site in the heart. In this condition of introduction and delivery, the stent must have an outside diameter of not more than approximately 1 mm for sufficient mobility. Having reached the stenotic passage of the coronary vessel, the stent is durably expanded for the stenosis to be removed. By means of the catheter on which it has been introduced, the stent is radially expanded to a diameter of approximately 4 mm, which is accompanied with plastic deformation. To this end, the catheter is a balloon catheter, in which the lengthwise section which supports the stent is dilated in a manner similar to a balloon by the application of superpressure by means of salt solution.
Conventional implanted stents consist of a metal material suitable for the medical use and which may be provided with an anticoagulant layer for the avoidance of thrombosis problems. A drawback of these durably implanted stents resides in the detectable permanent irritation of the tissue surrounding the stent, since the stent, because of its rigidity, does not perform the flexions, caused by heartbeat, of the coronary vessel it supports.
Furthermore, attention is drawn to the fact that as a rule the support by a stent for the expansion of a stenosis is required only for a period of some months. Afterwards, the part of the vessel affected by stenosis would remain open even without any support.
For the elimination of the afore-mentioned problems, stents have been proposed to be manufactured from bioresorbable materials which decompose in the human body in the course of few months. A method for the manufacture of such bioresorbable coronary stents is known for instance from U.S. patent application Ser. No. 08/733,172. In this case, a stent blank of homogeneous polymeric structure is built up from a viscous solution of poly-.beta.-hydroxybutanoic acid as a bioresorbable polymeric material in a solvent by successively coating a male mold core with layers of the polymer solution in several steps by precipitation of the polymeric material by the solvent being evaporated and by the layer previously precipitated being dissolved at least partially. This stent blank is then drawn off the male mold core and subsequently treated to finish the shaping of the stent.
It is true that bioresorbable coronary stents of polymeric materials have the desired biological resorbability and biocompatibility. However, problems are posed by the often insufficient mechanical properties such as a lack of plastic deformability of this stent. Upon dilatation to as much as four times the diameter, this will lead to fissuring with the consequence of reduced mechanical stability and a high degree of re-deformation. This means that for a final diameter of 4 mm, the maximum expansion must be clearly higher, which again leads to a further increase of fissuring accompanied with corresponding destabilization of the stent.